Method of making a metal article having a chromium bonded surface

ABSTRACT

A method of electrodepositing chromium upon the surface of magnetizable metal article to effect a bonding therewith and a diffusion therethrough of chromium to increase the surface hardness, and particularly the scratch hardness thereof, and the metal article so produced. The method includes the steps of passing a current of sufficient strength and density through a chromium plating bath to effect a penetration of chromium into a magnetized metal article as the cathode. Magnetization of the article may be effected prior to the electrodeposition step so as to leave the metal article with residual magnetism when made the cathode in the electrodeposition bath. The composition of the electrolyte used preferably contains boric acid, in addition to chromic acid and a source of sulfate ions such as is customarily used in chromium electroplating baths.

United States Patent [72] Inventor Louis F. Ranieri 4036 North Central Ave., Chicago, 111. 60635 [21] Appl. No. 711,237 [22] Filed Mar. 7, 1968 [45] Patented Oct. 26, 1971 Continuation-impart of application Ser. No. 638,676, Apr. 26, 1967, now abandoned.

[ 54] METHOD OF MAKING A METAL ARTICLE HAVING A CHROMIUM BONDED SURFACE 4 Claims, 9 Drawing Figs.

[52] U.S. Cl 204/29 [51] Int.Cl C23b 5/62 [50] Field of Search 204/16, 56, 297 M, 29, 43

[56] References Cited UNITED STATES PATENTS 2,020,117 11/1935 Johnston 204/297 2,911,347 11/1959 Gutzmer 204/297 3,032,487 5/1962 Yonezaki et a1. 204/56 Primary Examiner-John H. Mack Assistant Examiner-W. 1. Solomon Attorney-Hill, Sherman, Meroni, Gross & Simpson ABSTRACT: A method of electrodepositing chromium upon the surface of magnetizable metal article to effect a bonding therewith and a diffusion therethrough of chromium to increase the surface hardness, and particularly the scratch hardness thereof, and the metal article so produced. The method includes the steps of passing a current of sufficient strength and density through a chromium plating bath to effect a penetration of chromium into a magnetized metal article as the cathode. Magnetization of the article may be effected prior to the electrodeposition step so as to leave the metal article with residual magnetism when made the cathode in the electrodeposition bath. The composition of the electrolyte used preferably contains boric acid, in addition to chromic acid and a source of sulfate ions such as is customarily used in chromium electroplating baths.

METHOD OF MAKING A METAL ARTICLE HAVING A CHROMIUM BONDED SURFACE CROSS-REFERENCE TO RELATED APPLICATION The instant application is a continuation-in-part of my application entitled, Metallurgically Bonded Article and Method of Making the Same," Ser. No. 634,676, filed Apr, 28, 1967 and now abandoned. The present application describes and claims an improvement.

BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, a magnetizable article, and more particularly a cutting tool of tool steel or other ferrous metal alloys customarily used for the purpose, is made the cathode while in a magnetized state in a chromium electrodeposition bath that preferably contains a source of borate anions (B furnished, most readily, by the addition of boric acid to the electrolyte. The bath is operated under much the same conditions as those used for hard chromium plating, using the approximate ratio of CrO to H 80, of 100 to l, but controlling the temperature of the electrolytei'ather more closely than in hard chromium plating to within the range of from 130 to 140, and, more preferably, within the narrow range of from 130 to 136 F. In addition to the chromic acid and sulfuric acid, an effective amount of boric acid, or other source of borate anions, is added to the bath, for example from one-quarter to 3 ounces of boric acid per gallon of electrolyte containing 33 ounces of chromic acid and 0.33 ounces of sulfuric acid. After being made up, the bath is preferably allowed to stand for 2 to 3 days in order to reach equilibrium, and the clear electrolyte is then separated from any sediment or precipitate and used in making up the bath.

In the operation of the bath, with the work piece magnetized and the south pole of the workpiece preferably uppermost and connected to the negative side of the outside source of DC electricity, and with an insoluble anode, such as lead, connected to the positive side of the same outside source of DC electricity, the current is turned on and the bath operated for a period of time varying from a few seconds up to as long as from 10 to l minutes, but generally less that 30 minutes. The current will generally be within the range of from 100 to 150 amps per square foot, and the voltage within the range of from 3 to volts, but current densities and voltages outside of the ranges can be employed depending upon the composition of the workpiece, special tool alloys requiring a general higher voltage.

After electrodeposition has been completed, if an effective amount of borate anions were present in the bath, the workpiece will be found to have a surface coating of almost pure chromium of a mean thickness of about 35 microns, with the outer 10 to microns of the deposit containing some boron that is believed to be in the form of a boride, such as chromium boride. Quite surprisingly, when the same workpiece, not magnetized, is subjected under similar conditions as to composition of electrolyte, current density, voltage and time of operation of the bath, it will show upon microprobe analysis much less chromium in the electrodeposit on the surface of the workpiece and no boron in the electrodeposit. While some of the benefits of my invention are attained without the use of the magnetizing step, or with the magnetizing step but without the presence of the borate anion in the electrolyte, greatly superior results as to scratch hardness of the coated, working surface of the tool are obtained when both the magnetizing step and the addition of boric acid to the electrolyte are employed. Surprisingly, when these conditions are employed there is no measurable increase in the dimension of the workpiece as would be the case if a hard chromium plate were formed on the surface of the workpiece by conventional chromium plating methods.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic view of an electrodeposition bath illustrating the arrangement of the magnetized workpiece as the cathode and of an inert anode connected to the positive side of an outside source of DC electricity;

FIGS. 2, 3, and 4 are elevational views of various types of workpieces, such as a drill bit, a burr, and a saw blade, respectively;

FIG. 5 is an illustration of an edge area of a tool in which the section has been subjected to chromium X-rays, 250 magnification;

FIG. 6 is a similar sectional area including the edge portion subjected to iron X-rays, 250 magnification;

FIG. 7 is a section of a workpiece not subjected to magnetization but otherwise processed the same as the workpieces of FIGS. 5 and 6, subjected to chromium X-rays, 250 magnification;

FIG. 8 is a chart of the results of a microprobe analysis of the coating of the magnetized workpiece of FIG. 5, showing the percent of chromium (Cr) as the ordinate, and the distance from the surface in microns 1,); and

FIG. 9 is a similar chart but made on a workpiece not subjected to magnetization and therefore comparable with the workpiece from which the picture of FIG. 7 was made.

DETAILED DESCRIPTION As best shown in FIG. 1, the method of my invention is suitably carried out in a plating tank 10 containing a bath ll of an electrolyte having a composition that will be described hereinafter. An outside source of DC electricity, such as battery 12, connected through a lead 13 to a bus bar 14 that is indicated by the plus sign as the positive side of the electrical circuit. The other side of the battery 12 is connected through a lead line 15 to a bus bar I6 which, is indicated by the negative sign is the negative side of the electrical circuit. In place ofthe battery, there may be employed a rectifier, such as a ripple free" rectifier, connected to a source of AC electricity. In order to maintain the electrolyte of the bath II at a proper temperature, preferably between and F., and even more desirably between I30 and 134 F., a heating coil 17 is provided near the bottom of the tank 10, with an inlet 18 for the introduction of stream thereinto and with an outlet 19 for the discharge of condensate. As is customary, the amount of heat furnished by the heating element or coils I7 is controlled thermostatically in accordance with the temperature of the electrolyte.

In FIG. 1, the workpiece, as indicated by reference numeral 20, is made the cathode, and is held submerged, or partially submerged, in the electrolyte bath 11 by means of a hook or hanger 21 supported from the bus bar 16 and in electrical contact therewith. The anode, represented by the reference numeral 22 is suspended by means of the hanger of the like 23 from the bus bar 14, so as to be immersed or substantially so, within the electrolyte of the bath ll. Ordinarily, a lead anode is employed, the same as in conventional chromium plating operations, and the lead may be in the form of a strip having plane surfaces or corrugated surfaces.

In accordance with my invention, the workpiece 20, prior to being made the cathode in the bath 11, is subjected to magnetization as by insertion within a magnetizing coil, so that the working end of the workpiece is in the north pole N and the other end in the south pole S, and when inserting the workpiece 20 in the electrolyte, the portion of the workpiece representing the south pole is connected through the hanger 21 to the bus bar 16. My method is operative with the poles reversed from the arrangement show in FIG. I, but better results have been realized by making the uppermost portion of the workpiece the south pole and connecting that to the negative side of the external circuit. The extent to which the workpiece is magnetized does not seem to be critical, provided that the workpiece 20 when immersed in the electrolyte of the bath 11 has sufficient residual magnetization to attract the opposite pole ofa compass. Thus by, simply testing with a compass, one can determine that the upper end of the workpiece 20 is the south pole, since it will attract the north pole of the compass,

and conversely, the working end, which is the lowermost end of the workpiece 20 when suspended as a cathode, will constitute the north pole and therefore attract the south pole of the compass. In magnetizing an elongated workpiece, such as the drill bit of FIG. 2, indicated by the reference numeral 25, or the burr 26 of FIG. 3, the magnetizing forces act axially of the workpiece, whereas in the case of a new blade 27, as illustrated in FIG. 4, the blade is magnetized across its width, rather than its length, so as to make the nonworking edge of the saw blade the south pole, and working edge of the saw blade, indicated by the reference numeral 28, the north pole.

The workpiece is preferably of a magnetizable composition, such as a ferrous metal alloy, but in order to achieve the best results from the standpoint of obtaining a cutting edge of high effectiveness over a long period of time, the workpiece is most suitably formed of an alloy steel, such as a high-speed tool steel. Other compositions than ferrous metals can be used, even though not magnetizable, so long as they can be polarized under the influence of an applied magnetic field, but the magnetizable metal compositions that are used in tools for cutting, drilling, burring and the like are the most suitable ones for processing in accordance with the method of the present invention.

As the electrolyte used in making up the bath 1], I prefer to make up a solution entirely similar to that used as the electrolyte in the electrodeposition of hard chromium, but modified by the inclusion therein of an effective amount of boric acid, or other source of borate (BO anions. As typical of the electrolyte composition that I employ, the following composition is illustrative:

Chromium (Cr0,) Sulfuric Acid Boric Acid (H80 Since the preferred ratio of chromium calculated as (Cro to sulfuric acid is 100 to l, the preferred composition of electrolyte contains 33 oz. of chromium and 0.33 oz. of sulfuric acid per gallon. The preferred range of boric acid is from about 0.5 to 2.5 oz. per gallon, and this proportion, along with the proportion of chromium to sulfuric acid, should be maintained during the use of the bath by suitable additions of the respective chemicals, or of water to replace water lost by evaporation.

After the electrolyte has been prepared, it should be allowed to stand for 2 to 3 days in order to permit any solids in the electrolyte to precipitate out. Preferably, the water used is distilled water and the chromic acid and boric acid are of a high degree of purity, such as represented by the technical grades of the respective chemicals. However, it has been found best to allow the freshly made electrolyte to stand for a sufficient length of time to reach equilibrium and to allow any sedimentation to form if there are undissolved solid constituents in the bath. After standing for the length of time indicated, the clear electrolyte is separated from the sediment or precipitation, as by decanting, and is used in making up the bath 1 1.

As to the conditions under which the bath I1 is operated, these will depend to some extent upon the compositions and dimensions of the bath and workpiece, respectively, and upon the result desired. In the case of a high-speed tool, such as the drill bit 25, the current through the bath will be maintained at approximately 4 volts and from 7 to 8 amperes per square foot, and the electrodeposition will be carried out over a period of approximately 55 seconds. The amperage per square foot will, of course, vary with the surface area undergoing electrodeposition, but, in general, will vary between about 100 to 150 amps per square foot, or higher. In the case ofa l-inch end mill, 2 inches of the length of which is to be provided with an electrodeposit in accordance with my method, the voltage may be between 4% and volts, the applied current being between 40 and 60 amperes, and the length of electrolytic treatment about 50 seconds, while with l-inch end mill, the

voltage may be from 4 to 4%, the applied amperage between 20 and 30 amperes and the length of processing may be about 45 seconds.

Prior to the application of an electrodeposit upon a workpiece, the workpiece, of course, is properly cleaned. This can be accomplished by any of the usual practices, including the use of a solvent degreaser; a preliminary treatment with an alkali wash, followed by an electrolytic cleaning step, either anodic or cathodic; or merely making the workpiece the anode in the electrolytic bath for a sufficient length of time to clean the surface of the workpiece and thereafter reversing the current so that the workpiece is the cathode, as shown in FIG. 1. It is generally not desirable to reverse the current during the period of electrodepositing chromium upon the workpiece. The workpiece can be magnetized either before or after the cleaning step, or magnetizing forces can be applied to the workpiece while carrying out the electrodeposition step, or just prior thereto. The intensity of the magnetic field used in magnetizing the workpiece can be in the neighborhood of 600 gauss, but any intensity of the magnetizing field that will accomplish the polarization of the workpiece is in general sufficient.

The results of microprobe analyses are illustrated in FIGS. 5, 6, and 7. In FIG. 5, the picture there reproduced shows the sectional area, including the edge, of the workpiece that has been subjected to chromium X-rays at a magnification of 250. The bright area is the chromium plated area while the darker, spotted area is the base metal containing about 5.3 percent of chromium. The workpiece in the illustration of FIG. 5 is one that has been subjected to the preferred process as herein described, including magnetization and electrodeposition from a bath containing chromium and boric acid within the specified ranges.

The picture of FIG. 6 is also that of the same area as illustrated in FIG. 5, but using iron X-ray at the same magnification of 250. This illustration shown in FIG. 6 confirms the fact that there is no iron in the plating area.

FIG. 7 is a picture of a similar area, including the edge, of a section of a workpiece that has not been magnetized, but that has otherwise been subjected to the same electrodeposition step as the workpieces of FIGS. 5 and 6. Under chromium X- rays at 250 magnification, there is only a slight increase in chromium along the edge, which is the upper, lighter portion as viewed in FIG. 7.

The following data were obtained in the case of three drills, similar to the drill 25, which were first weighed in the unplated condition, then plated and weighed again, and then, after removing the free chromium in the coat by dissolving it in 50 percent concentration of hydrochloric acid, was again weighed. The weight losses due to dissolution of the chromium by the hydrochloric acid would have been much greater if all ofthe chromium in the sample had been free chromium. From this, applicant concludes that some of the chromium must be present in the form of a boride. The following table is a compilation of the weights of the three drill bits when processed as just described:

Each of the three drill bits in the foregoing table was subjected to the process of my invention, including the magnetization step and the use of boric acid in the electrolyte, maintaining all conditions of processing substantially identical.

In FIG. 8, which represents the results ofa microprobe analysis of a workpiece process in accordance with the preferred method of my invention, the curve A represents the percent of chromium as the ordinate plotted against the distance from the surface in microns as the abscissa. This chart shows that the workpiece was thickly coated with almost pure chromium, the mean thickness of the coat being about 35 microns. The outer to microns of plating contains some boron which is represented by the line B. The inner face of the plate in contact with th e steel 6r other foundation metal, shows some carbon and a trace of nitrogen. The graph of FIG. 8 shows the average chromium composition across the depth of the chromium electrodeposit, including penetration of chromium beyond the plated chromium. It also shows the extent of penetration of the boron in whatever form it may be present, to a depth of something over 15 microns.

FIG. 9, is a graph similar to that of FIG. 8, but represents the microprobe analysis of a similar workpiece that had not been magnetized, but had been subjected to the same conditions otherwise, including electrodeposition from a chromium bath containing boric acid. This chart, as shown by the curved line C, exhibits much less chromium in the plating, the total thickness of plating being only about -25 microns, and the maximum chromium concentration in the plating area being only about 75 percent, as against the nearly 90 percent shown in the line A in FIG. 8. There appears, however, to be muchv more diffusion of the chromium in the workpiece represented by FIG. 8 than in that represented by FIG. 9. Without the magnetization, as is apparent from the chart of FIG. 9, no boron was found.

lclaim: I l. A method of electrodepositing a hard boron-containing chromium layer on the surface of a magnetizable article of a ferrous metal alloy, which comprises effecting the electrodeposition by making said article while magnetized the cathode in an aqueous chromium plating bath containing from about 0.16 to 0.33 oz. per gal. of sulfuric acid and from one-fourth to 3 oz. per gal. of (B0 ions calculated as boric acid (H and maintaining said bath at a temperature of between and F. while passing through said bath over a period of time an electric current of a voltage and current density such as to deposit a surface layer containing some boron to increase the scratch hardness of said surface layer.

2. The method of claim 1, wherein said article is an elongated tool suspended in said plating bath with its longer dimension extending vertically and with its uppermost portion constituting the south pole.

3. The method of claim 1, wherein said article is a tool having its cutting surface forming the cathode immersed in said bath, and

having a noncutting surface magnetized as the south pole of the tool.

4. The method of claim I, wherein said electrodeposition is carried out for from a few seconds up to 30 minutes while said article is magnetized as a result of applying thereto a magnetic field of a force intensity in the neighborhood of 600 gauss. 

2. The method of claim 1, wherein said article is an elongated tool suspended in said plating bath with its longer dimension extending vertically and with its uppermost portion constituting the south pole.
 3. The method of claim 1, wherein said article is a tool having its cutting surface forming the cathode immersed in said bath, and having a noncutting surface magnetized as the south pole of the tool.
 4. The method of claim 1, wherein said electrodeposition is carried out for from a few seconds up to 30 minutes while said article is magnetized as a result of applying thereto a magnetic field of a force intensity in the neighborhood of 600gauss. 